Vertical alignment type liquid crystal display device with viewing angle characteristics improved by disposing optical plates

ABSTRACT

First and second polarizers are disposed in cross-Nichol configuration. A liquid crystal cell is disposed between the two polarizers and establishes vertical alignment in a state of no voltage application. An even number of optical films having optical anisotropy and disposed between the liquid crystal cell and first polarizer. A retardation of the liquid crystal cell is in a range between 300 nm and 1500 nm; and each optical film satisfies nx&gt;n≧z, an in-plane retardation is smaller than 300 nm, a thickness direction retardation is in a range between 50 nm and 300 nm, an angle between an in-plane slow axis of the optical film disposed nearest to the first polarizer and an absorption axis of the first polarizer is smaller than 45°, and the slow axes of mutually adjacent optical films are perpendicular to each other.

CROSS REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 12/367,680, filed Feb.9, 2009 now U.S. Pat. No. 8,199,283, which is based upon and claims thebenefit of priority from Japanese Patent Applications Nos. 2008-046597,2008-046599 and 2008-081583, filed on Feb. 27, 2008, Feb. 27, 2008 andMar. 26, 2008 respectively, the entire contents of all of which areincorporated herein by reference.

FIELD

The present invention relates to a liquid crystal display device havingvertically aligned liquid crystal molecules and a pair of cross-Nicholdisposed polarizers.

BACKGROUND

As an in-vehicle display device, a liquid crystal display device capableof reducing the weight and using a battery equipped in a vehicle withoutchange has been paid attention. There is a need of lowering brightnessof a background display area and a dark display area in order to enhanceexpensive looking. A normally black type liquid crystal display unit hasbeen developed which uses a light emitting diode as a light source of abacklight to emit light generally at a single wavelength and increase acontrast dramatically only in this wavelength range.

A display device utilizing a vertical alignment mode (VA mode) liquidcrystal cell has been paid attention as a liquid crystal display devicecapable of realizing high-quality normally black display independentfrom an emission wavelength of a backlight. In a VA mode liquid crystalcell, liquid crystal molecules are vertically aligned relative to asubstrate surface when voltage is not applied or off-voltage is applied(collectively called “in an off state” in some cases). “Verticallyaligned” does not mean that a direction of a director of liquid crystalmolecules is strictly vertical to the substrate surface, but it meansthat the director of liquid crystal molecules is aligned along adirection nearly vertical to the substrate surface as compared to thatthe director is inclined from a substrate normal direction when voltageis applied (called “in an on state” in some cases). This liquid crystalcell is disposed between two polarizers in approximately cross-Nicholconfiguration to constitute a liquid crystal display unit and realizenormally black display.

The optical characteristics of a vertical alignment mode liquid crystalcell are approximately isotropic when observed along a substrate normaldirection, and the optical characteristics of a liquid crystal displayunit are influenced by the optical characteristics of polarizers incross-Nichol configuration. A transmittance becomes therefore very low,and a high contrast can be realized relatively easily. However, whenobserved along an oblique direction, optical leak occurs in the blackdisplay state. This is because birefringence occurs in the liquidcrystal layer, and the transmission axes of two polarizers shift fromthe perpendicular relation. In order to suppress a contrast, whenobserved along an oblique direction, from being lowered, the followingvarious methods have been proposed.

JP-SHO-62-210423 discloses a liquid crystal display unit having aviewing angle compensator having negative uniaxial optical anisotropy ornegative biaxial optical anisotropy inserted at one or both positionsbetween a liquid crystal cell and two polarizers. The positive opticalanisotropy of a liquid crystal cell in a thickness direction iscompensated by the viewing angle compensator having the negative opticalanisotropy in a thickness direction. The “viewing angle compensatorhaving the negative biaxial optical anisotropy” means a viewing anglecompensator having a relation of nx>ny>nz where nx, ny and nz are x-, y-and z-components of a refractive index in which an x-axis is a slow (lagphase) axis direction in an in-plane of a substrate or film, a y-axis isa fast (advance phase) axis direction and a z-axis is a thicknessdirection. The “viewing angle compensator having the negative uniaxialoptical anisotropy” means a viewing angle compensator having a relationof nx=ny>nz. JP-A-2000-131693 discloses the effective conditions for anin-plane phase difference and an arrangement of the azimuth of anin-plane slow axis of a viewing angle compensator having biaxial opticalanisotropy.

A viewing angle compensator having negative uniaxial optical anisotropyis called a “negative C plate”. A viewing angle compensator havingnegative biaxial optical anisotropy is herein called a “negative biaxialfilm”. A viewing angle compensator having positive uniaxial opticalanisotropy with its slow axis being oriented in an in-plane direction,i.e., an optical film having a relation of nx>ny=nz, is called a“positive A plate”. The positive A plate can be considered as a specialexample wherein the refractive indices ny and nz of a negative biaxialfilm are equal to each other.

JP-A-2000-39610 discloses a method of using an approximatelyhalf-wavelength plate having biaxial optical anisotropy, and a negativeC plate. With this method, since the half-wavelength plate is requiredto provide a phase difference of approximately a half wavelength whenobserved along any direction, a half wavelength plate having positivebiaxial optical anisotropy is required in practical uses. However, it isdifficult to realize a half-wavelength plate having positive biaxialoptical anisotropy.

JP-A-2003-262869 discloses a method of using a combination of a negativebiaxial film and a negative C plate. With this method, an in-planeretardation of the biaxial film is limited to 190 nm or smaller, and aretardation of a liquid crystal layer is limited to 200 to 500 nm. Aretardation of a liquid crystal layer is represented by Δnd where Δn isrefractive index anisotropy of liquid crystal material and d is athickness of the liquid crystal layer.

SUMMARY

It is known that an increase in a retardation of a liquid crystal cellis effective for promoting steepness of a change in a transmittance withrespect to a change in voltage applied to a liquid crystal displaydevice. In order to multiplex-drive a VA mode liquid crystal displayunit at a duty of ¼ to 1/240, it is preferable to set a retardation Δndof a liquid crystal layer larger than 320 nm, and more preferably largerthan 360 nm. This is because as a retardation of the liquid crystallayer becomes small, it becomes difficult to maintain both the highcontrast characteristics, which is a feature of a normally black type VAmode liquid crystal display unit, and a high transmittance during anon-voltage application in high duty driving.

Negative biaxial films are distributed in markets which are formed byexecuting a biaxial stretching process for a base film made ofnorbornene based cyclic olefin polymer (hereinafter described as“norbornene based COP”) or by executing a stretching process for atriacethyl cellulose (hereinafter described as “TAC”) base film.

From the viewpoint of ensuring in-plane uniformity of a retardation,generally a negative biaxial film made of norbornene based COP hasranges of an in-plane retardation Re of 30 nm to 300 nm, a thicknessdirection retardation Rth of 300 nm or smaller and an Nz factor of 1 to12. Re, Rth and Nz are given by Re=(nx−ny)/d, Rth=((nx+ny)/2−nz)×d, andNz=(nx−nz)/(nx−ny), where nx is a refractive index in an in-plane slowaxis direction, ny is a refractive index in an in-plane fast axisdirection, nz is a refractive index in a thickness direction and d is athickness. It is difficult to ensure in-plane uniformity of aretardation of a biaxial film having Re, Rth and Nz in excess of theabove-described ranges.

The ranges of a retardation and Nz factor of a TAC based biaxial filmdistributed in markets are narrower than a biaxial film using norbornenebased COP. Generally, an in-plane retardation Re is in a range between40 nm and 70 nm, and a thickness direction retardation Rth is in a rangebetween 120 nm and 220 nm.

An ideal C plate has an in-plane retardation Re of 0, whereas anin-plane retardation Re of a C plate actually distributed in markets isnot strictly 0. An in-plane retardation Re of a general C plate is setpreferably to 7 nm or smaller, and more preferably to 5 nm or smaller. Cplates widely distributed in markets are TAC films having a thicknessdirection retardation Rth of about 50 nm, and it is difficult topurchase C plates having a different thickness direction retardationRth.

A range of a retardation of a liquid crystal layer is restricted whenviewing angle compensation for a normally black type VA mode liquidcrystal display unit is performed by a conventional viewing anglecompensation method using a negative biaxial film and a negative C plategenerally available in markets.

An object of the present invention is to provide a liquid crystaldisplay unit capable of having a wide retardation range of a liquidcrystal cell whose viewing angle can be compensated by using readilyavailable optical anisotropic films.

According to one aspect of the present invention, there is provided aliquid crystal display device comprising:

first and second polarizers mutually cross Nichol disposed;

a liquid crystal cell disposed between the first and second polarizersand establishing vertical alignment in a state of no voltageapplication; and

an even number of optical films having optical anisotropy and disposedbetween the liquid crystal cell and the first polarizer,

wherein:

a retardation of the liquid crystal cell is not smaller than 300 nm andnot larger than 1500 nm; and

each of the optical films satisfies nx>ny≧nz where nx, ny and nz are x-,y- and z-components of a refractive index in which an x-axis is anin-plane slow axis azimuth of each of the optical films, a y-axis is anin-plane azimuth perpendicular to the x-axis and a z-axis is an azimuthperpendicular to a film surface, an in-plane retardation is not largerthan 300 nm, a thickness direction retardation is not smaller than 50 nmand not larger than 300 nm, an angle between an in-plane slow axis ofthe optical film disposed nearest to the first polarizer and anabsorption axis of the first polarizer is not larger than 45°, and theslow axes of mutually adjacent optical films are mutually perpendicular.

According to another aspect of the present invention, there is provideda liquid crystal display device comprising:

first and second polarizers mutually cross Nichol disposed;

a liquid crystal cell disposed between the first and second polarizersand establishing vertical alignment in a state of no voltageapplication; and

an odd number of three or more optical films having optical anisotropyand disposed between the liquid crystal cell and the first polarizer,

wherein:

a retardation of the liquid crystal cell is not smaller than 550 nm andnot larger than 1500 nm; and

each of the optical films satisfies nx>ny≧nz where nx, ny and nz are x-,y- and z-components of a refractive index in which an x-axis is anin-plane slow axis azimuth of each of the optical films, a y-axis is anin-plane azimuth perpendicular to the x-axis and a z-axis is an azimuthperpendicular to a film surface, an in-plane retardation is not smallerthan 30 nm not larger than 300 nm, a thickness direction retardation isnot smaller than 50 nm and not larger than 300 nm, an angle between anin-plane slow axis of the optical film disposed nearest to the firstpolarizer and an absorption axis of the first polarizer is not smallerthan 45° and not larger than 135°, and the slow axes of mutuallyadjacent optical films are mutually perpendicular.

According to another aspect of the present invention, there is provideda liquid crystal display device comprising:

first and second polarizers mutually cross Nichol disposed;

a liquid crystal cell disposed between the first and second polarizersand establishing vertical alignment in a state of no voltageapplication; and

a first optical film disposed between the liquid crystal cell and thefirst polarizer and having optical anisotropy;

a second optical film disposed between the first optical film and theliquid crystal cell and having optical anisotropy; and

a third optical film disposed between the second polarizer and theliquid crystal cell and having optical anisotropy;

wherein:

each of the first to third optical films satisfies nx>ny≧nz where nx, nyand nz are x-, y- and z-components of a refractive index in which anx-axis is an in-plane slow axis azimuth of each of the optical films, ay-axis is an in-plane azimuth perpendicular to the x-axis and a z-axisis an azimuth perpendicular to a film surface, an in-plane retardationis not smaller than 0 nm and not larger than 300 nm, and a thicknessdirection retardation is not smaller than 50 nm and not larger than 300nm;

a slow axis of the first optical film is parallel to an absorption axisof the first polarizer;

a slow axis of the second optical film and a slow axis of the thirdoptical film are mutually perpendicular; and

an angle between an in-plane axis of the second optical film and anabsorption is 75° to 105°.

According to another aspect of the present invention, there is provideda liquid crystal display device comprising:

a liquid crystal cell establishing vertical alignment in a state of notvoltage application;

first and second polarizers mutually cross Nichol disposed andsandwiching the liquid crystal cell;

a first optical film disposed between the liquid crystal cell and thefirst polarizer and having optical anisotropy;

a second optical film disposed between the first optical film and theliquid crystal cell and having optical anisotropy;

a third optical film disposed between the second polarizer and theliquid crystal cell and having optical anisotropy; and

a fourth optical film disposed between the third optical film and thesecond polarizer and having optical anisotropy,

wherein:

each of the first to fourth optical films satisfies nx>ny≧nz where nx,ny and nz are x-, y- and z-components of a refractive index in which anx-axis is an in-plane slow axis azimuth of each of the optical films, ay-axis is an in-plane azimuth perpendicular to the x-axis and a z-axisis an azimuth perpendicular to a film surface, and the first to fourthoptical films have approximately same optical characteristics;

a slow axis of the first optical film is approximately parallel to anabsorption axis of the first polarizer;

a slow axis of the first optical film is approximately perpendicular toa slow axis of the second optical film;

a slow axis of the fourth optical film is approximately parallel to anabsorption axis of the second polarizer; and

a slow axis of the third optical film is approximately perpendicular toa slow axis of the fourth optical film.

By disposing a plurality of optical films having the above-describedoptical anisotropy, it becomes possible to adopt readily availableoptical films.

By disposing two optical films having the above-described opticalanisotropy on each side, it becomes possible to broaden the rangecapable of compensating a viewing angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a reference example.

FIG. 2 is a graph showing a relation between an in-plane retardation anda transmittance of an optical film used in the liquid crystal displayunit of the reference example.

FIG. 3 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to another reference example.

FIG. 4 is a graph showing a relation between an in-plane retardation anda transmittance of a negative biaxial film used in the liquid crystaldisplay unit of the other reference example.

FIG. 5 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a first embodiment.

FIGS. 6A to 6E are schematic diagrams showing an example of the azimuthsof in-plane slow axes of two optical films used in the optical displayunit shown in FIG. 5.

FIGS. 7A to 7C are graphs showing viewing angle characteristics of theliquid crystal display unit shown in FIG. 5.

FIGS. 8A to 8C are graphs showing the relation between an azimuth of anin-plane slow axis and a transmittance of a first optical film of theliquid crystal display unit shown in FIG. 5.

FIG. 9 is a graph showing the viewing angle characteristics of theliquid crystal display unit shown in FIG. 5.

FIG. 10 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a second embodiment.

FIG. 11 is a graph showing the viewing angle characteristics of theliquid crystal display unit shown in FIG. 10.

FIG. 12 is a graph showing the relation between an azimuth of anin-plane slow axis and a transmittance of a second optical film of theliquid crystal display unit shown in FIG. 10.

FIG. 13 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a third embodiment.

FIG. 14 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a fourth embodiment.

FIG. 15 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm of the liquid crystal display unit of the fourth embodiment.

FIGS. 16A to 16D are graphs showing simulated equi-transmittance curvesof the liquid crystal display unit of the fourth embodiment.

FIG. 17 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm when in-plane retardations of first to third optical films of theliquid crystal display unit of the fourth embodiment are set equal.

FIG. 18 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm when in-plane retardations of second and third optical films of theliquid crystal display unit of the fourth embodiment are set equal andthe in-plane retardation of the first film is changed.

FIG. 19 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm when in-plane retardations of first and second optical films of theliquid crystal display unit of the fourth embodiment are set equal andthe in-plane retardation of the third optical film is changed.

FIG. 20 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm when in-plane retardations of first and third optical films of theliquid crystal display unit of the fourth embodiment are set equal andthe in-plane retardation of the second optical film is changed.

FIG. 21 is a graph showing simulation results of the relation between anazimuth of an in-plane slow axis and a transmittance of a second opticalfilm of a liquid crystal display unit according to a fifth embodiment.

FIG. 22 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to a sixth embodiment.

FIG. 23 is a graph showing simulation results of the relation between aretardation and a transmittance of each liquid crystal cell of theliquid crystal display devices of the fourth, sixth and seventhembodiments.

FIG. 24 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to the seventh embodiment.

FIG. 25 is a graph showing simulation results of the relation between anazimuth of the in-plane slow axis and a transmittance of a fifth opticalfilm of the liquid crystal display unit of the seventh embodiment.

FIG. 26 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to an eighth embodiment.

FIG. 27 is a graph showing simulation results of a change in atransmittance (optical leak) during an off state relative to a change inan observation direction from right and left from a substrate normaldirection, of each sample having a different slow axis of an opticalfilm of the liquid crystal display unit of the eighth embodiment.

FIG. 28 is a graph showing simulation results of a change in atransmittance during an off state relative to a change in a slow axisazimuth of an optical film on the polarizer side.

FIG. 29 is a graph showing simulation results of a change in atransmittance similar to that shown in FIG. 5 when Re is set to 30 nmfor the liquid crystal display unit of the eighth embodiment.

FIG. 30 is a graph showing simulation results of a change in atransmittance similar to that shown in FIG. 5 when Re is set to 70 nmfor the liquid crystal display unit of the eighth embodiment.

FIG. 31 is a graph showing simulation results of a change in atransmittance (optical leak) relative to a change in a viewing anglewhen Re of a second optical film of the liquid crystal display unit ofthe eighth embodiment.

FIG. 32 is a graph showing simulation results of a change in atransmittance relative to a viewing angle in the case of Δnd=1500 nm andRth=300 nm of the liquid crystal display unit of the eighth embodiment.

FIG. 33 is a block diagram of a liquid crystal display device.

DESCRIPTION OF EMBODIMENTS

Description will now be made on preferred conditions for improving theviewing angle characteristics in black state of a normally black type VAmode liquid crystal display unit, by using a conventional optical filmarrangement.

FIG. 1 is a schematic diagram showing a conventional normally black typeVA mode liquid crystal display unit. A liquid crystal cell 20 isdisposed between a rear side polarizer 10 and a front side polarizer 30in cross-Nichol configuration. A negative biaxial film 15 is disposedbetween the rear side polarizer 10 and liquid crystal cell 20. Anothernegative biaxial film 25 is disposed between the front side polarizer 30and liquid crystal cell 20.

The liquid crystal cell 20 includes a pair of substrates 21 and 22 andliquid crystal material 23 held in a space between the substrates.Common electrodes and segment electrodes are formed on opposing surfacesof the substrates 21 and 22, respectively, and vertical alignment filmsare also formed on the opposing surfaces. The vertical alignment filmshave already been subjected to a rubbing process to obtain mutuallyantiparallel rubbing directions. This rubbing process may use the methoddisclosed, e.g., in JP-A-2005-234254.

A distance between the substrates 21 and 22 is adjusted by a sphericalspacer to set a distance of, e.g., 2 to 6 μm. A refractive indexanisotropy Δn of the liquid crystal material is in a range between 0.08and 0.25, and a dielectric constant anisotropy Δ∈ is negative. A pretiltangle (angle between a director of liquid crystal molecules and asubstrate surface) is about 89.9°. Since liquid crystal molecules areoriented generally vertically to the substrate surface, the liquidcrystal layer is isotropic when observed along a substrate normaldirection. After the liquid crystal material 23 is injected between thesubstrate 21 and 22, the liquid crystal layer is cured for one hour at atemperature higher by about 20° C. than an isotropic phase temperatureto obtain the liquid crystal cell 20.

As the rear side polarizer 10 and front side polarizer 30, SHC13Umanufactured by Polatechno Co. Ltd. may be used. The rear side polarizer10 is constituted of a TAC base film 12 and a polarizing layer 11 formedon the surface of the TAC base film 12. The front side polarizer 30 isconstituted of a TAC base film 31 and a polarizing layer 32. The TACbase films 12 and 31 are disposed at positions inner than the polarizinglayers 11 and 32 (on the liquid crystal cell 20 side). An in-planeretardation Re of each of the TAC base films 12 and 31 is 3 nm, and athickness direction retardation Rth is 50 nm. Outer surfaces of thepolarizing layers 11 and 32 are covered with a protective film made ofTAC or the like.

As the negative biaxial films 15 and 25, for example, a norbornene basedCOP film subjected to a biaxial stretching process may be used. Theoptical characteristics of both the films were set equal to each other.Namely, an in-plane retardation Re1 of the biaxial film 15 is equal toan in-plane retardation Ret of the biaxial film 25, and a thicknessdirection retardation Rth1 of the biaxial film 15 is equal to athickness direction retardation Rth2 of the biaxial film 25.

A backlight is disposed outside the rear side polarizer 10, and theliquid crystal display unit is visually observed from the side of thefront side polarizer 30.

Azimuth angles are defined in such a manner that in a state that theliquid crystal display unit is observed from the front side, right andleft are at 0° and 180° and up and down are at 90° and 270°,respectively. Azimuths of an absorption axis 11 a and an in-plane slowaxis 12 s of the rear side polarizer 10 are set to 45°, and azimuths ofan absorption axis 32 a and an in-plane slow axis 31 s of the front sidepolarizer 30 are set to 135°.

An in-plane slow axis 15 s of the negative biaxial film 15 disposed onthe rear side is set to an azimuth perpendicular to the absorption axis11 a of the nearby rear side polarizer 10, i.e., to an azimuth of 135°.An in-plane slow axis 25 s of the negative biaxial film 25 disposed onthe front side is set to an azimuth perpendicular to the absorption axis32 a of the nearby front side polarizer 30, i.e., to an azimuth of 45°.

An azimuth of pretilt of the liquid crystal material 23 of the liquidcrystal cell 20 is set to 90°.

The relation between a transmittance and an in-plane retardation Re ofthe biaxial film when observing the liquid crystal display unit in ablack state at a viewing angle of 45° is calculated by simulation, atretardations Δnd of 360 nm, 600 nm and 900 nm.

FIG. 2 shows simulation results. The abscissa represents retardationsRe1 and Re2 of the biaxial films 15 and 25 in the unit of “nm”, and theordinate represents a transmittance in the unit of “%”. Each of thethickness direction retardations Rth1 and Rth2 of the biaxial films 15and 25 is set to 110 nm, 220 nm and 300 nm at the retardations Δnd ofthe liquid crystal cell 20 of 360 nm, 600 nm and 900 nm, respectively.

A solid line in FIG. 2 indicates a transmittance when observed along avisual line inclined by 45° toward the azimuth of 180°, and a brokenline indicates a transmittance when observed along a visual lineinclined by 45° toward the azimuth of 0°. The reason why thetransmittance is different between the azimuths of 0° and 180° is thatthe pretilt angle of the liquid crystal cell 20 was set to 89.9°. If thepretilt angle is 90°, i.e., if the liquid crystal molecules are strictlyaligned vertically, the transmittances at the azimuths of 0° and 180°are equal to each other.

A contrast can be increased by selecting the in-plane retardations Re1and Re2 of the biaxial films 15 and 25 so as to minimize thetransmittance. It can be understood that the in-plane retardations Re1and Re2 minimizing the transmittance depend on the retardation Δnd ofthe liquid crystal cell 20. A biaxial film having a different in-planeretardation Re could be used for a liquid crystal cell having adifferent retardation. Generally, the optimum retardation Δnd of theliquid crystal cell 20 depends upon the multiplex driving dutyconditions, so the retardation Δnd of liquid crystal cell 20 isdetermined based on the duty condition. It is therefore necessary to usea negative biaxial film having different optical characteristics for aliquid crystal display unit by different duty conditions.

FIG. 3 is a schematic diagram showing a conventional liquid crystaldisplay unit which performs viewing angle compensation by using anegative biaxial film and a negative C plate. In the example shown inFIG. 3, an optical film is not disposed between the rear side polarizer10 and the liquid crystal cell 20, whereas a negative C plate 26 and anegative biaxial film 27 are sequentially disposed between the frontside polarizer 30 and liquid crystal cell 20 from the liquid crystalcell 20 toward the front side polarizer 30. An in-plane slow axis 27 sof the negative biaxial film 27 is perpendicular to an absorption axis32 a of the front side polarizer 30. Namely, an azimuth of the in-planeslow axis 27 s is 45°. Other structures are the same as those of theliquid crystal display unit shown in FIG. 1.

FIG. 4 shows results of the same simulation as those shown in FIG. 2. Athickness direction retardation Rth4 of the biaxial film 27 is set to220 nm. A thickness direction retardation Rth3 of the negative C plate26 is set to 0 nm, 220 nm and 440 nm at the retardations Δnd of theliquid crystal cell 20 of 360 nm, 600 nm and 900 nm, respectively. The Cplate having the thickness direction retardation Rth3 of 0 nm is atransparent plate having no optical anisotropy.

The transmittance is minimum at an in-plane retardation Re4 of thenegative biaxial film 20 of about 50 nm, irrespective of retardation Δndof the liquid crystal cell 20. It is therefore possible to adoptnegative biaxial films having the same optical characteristics for aplurality of liquid crystal display units having different retardationsΔnd of the liquid crystal cell 20. Further, the negative biaxial filmand C plate used for the liquid crystal display unit whose liquidcrystal cell 20 has a retardation Δnd of 360 nm or smaller can be usedin common. It is therefore possible to reduce cost.

The C plate 26 and negative biaxial film 27 are disposed between thefront side polarizer 30 and liquid crystal cell 20 in the example shownin FIG. 3. It has been confirmed from simulations that the similareffects can be obtained even if one of the C plate 26 and negativebiaxial film 27 is disposed between the front side polarizer 30 andliquid crystal cell 20 and the other is disposed between the rear sidepolarizer 10 and liquid crystal cell 20. A liquid crystal display unitwas really manufactured, and it has been confirmed that the simulationanalysis results are satisfied.

It is however difficult to realize the C plate 26 having an in-planeretardation Re of 0 in a wide range.

First Embodiment

FIG. 5 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to the first embodiment. No optical filmis disposed between a rear side polarizer 10 and a liquid crystal cell20. A first optical film 40 and a second optical film 41 are disposedbetween a front side polarizer 30 and liquid crystal cell 20. The firstoptical film 40 is disposed on the side of the front side polarizer 30,and the second optical film 41 is disposed on the side of the liquidcrystal cell 20. Other structures are the same as those of the liquidcrystal display unit shown in FIG. 1.

A negative biaxial film or a positive A plate is used as the firstoptical film 40 and second optical film 41. Namely, the first opticalfilm 40 and second optical film 41 have optical anisotropy satisfyingnx>ny≧nz. In-plane retardations Re5 and Re6 of the first optical film 40and second optical film 41 were both set to 50 nm. Viewing anglecharacteristics were obtained by simulation by various azimuths of anin-plane slow axis 40 s of the first optical film 40 and an in-planeslow axis 41 s of the second optical film 41.

FIGS. 7A to 7C show viewing angle characteristics with respect to theright/left directions at the retardations Δnd of the liquid crystal cellof 320 nm, 600 nm and 765 nm. The abscissa represents a viewing angle inthe unit of “degree (°)”, and the ordinate represents a transmittance inthe unit of “%”. The “viewing angle” is an angle between a normal of thesubstrate surface of the liquid crystal display unit and a visual line.In FIGS. 7A to 7C, a viewing angle inclined toward the azimuth of 0°(right side) has a positive sign, and a viewing angle inclined towardthe azimuth angle of 180° (left side) has a negative sign.

A thickness direction retardation Rth5 of the first optical film 40 anda thickness direction retardation Rth6 of the second optical film 41were equal to each other, and had 110 nm, 220 nm and 300 nm at theretardations Δnd of the liquid crystal cell of 320 nm, 600 nm and 765nm, respectively. The liquid crystal cell 20 in a black state functionsas nearly a positive C plate. The thickness direction retardations ofthe first optical film 40 and second optical film 41 are determinedmainly in such a manner that the function of the liquid crystal cell 20as a positive C plate is compensated.

FIG. 6A shows the case wherein azimuths of the slow axes 40 s and 41 sare both set to 45°. FIG. 6B shows the case wherein the azimuths of theslow axes 40 s and 41 s are se to 45° and 135°, respectively. FIG. 6Cshows the case wherein the azimuths of the slow axes 40 s and 41 s areset to 135° and 45°, respectively. FIG. 6D shows the case whereinazimuths of the slow axes 40 s and 41 s are both set to 135°. FIG. 6Eshows the case wherein the azimuths of the slow axes 40 s and 41 s areset to 0° and 90°, respectively.

Curves a to e in FIGS. 7A to 7C indicate transmittances of thearrangements shown in FIGS. 6A to 6E, respectively. As shown in FIG. 6C,irrespective of retardation Δnd of the liquid crystal cell, the bestviewing angle characteristics can be obtained by setting the azimuth ofthe slow axis 40 s of the first optical film 40 to 135° and setting theazimuth of the slow axis 41 s of the second optical film 41 to 45°.Further, as shown in FIG. 6E, relatively good viewing anglecharacteristics can be obtained by setting the azimuth of the slow axis40 s of the first optical film 40 to 0° and setting the azimuth of theslow axis 41 s of the second optical film 41 to 90°.

Further, as shown in FIG. 6D, irrespective of retardation Δnd of theliquid crystal cell 20, the viewing angle characteristics are worst bysetting the azimuths of the slow axis 40 s of the first optical film 40and the slow axis 41 s of the second optical film 41 both to 135°. Asshown in FIG. 6A, the viewing angle characteristics are relatively badby setting the azimuths of the slow axis 40 s of the first optical film40 and the slow axis 41 s of the second optical film 41 both to 45°.

It can be predicted from the above-described evaluation results that itis preferable that the slow axis 40 s of the first optical film 40 andthe slow axis 41 s of the second optical film 41 are made perpendicularto each other.

Viewing angle characteristics were obtained by simulation by changingthe azimuth of a slow axis while the slow axis 40 s of the first opticalfilm 40 and the slow axis 41 s of the second optical film 41 aremaintained perpendicular to each other.

FIGS. 8A to 8C show simulation results. The abscissa represents anazimuth of the slow axis 40 s of the first optical film 40 in the unitof “degree (°)”, and the ordinate represents a transmittance in the unitof “%”. FIGS. 8A to 8C show evaluation results at the retardations Δndof the liquid crystal cell 20 of 320 nm, 600 nm and 765 nm,respectively. In all cases, an azimuth of inclining a visual line wasset to 0°, and a viewing angle was set to 50°.

A thickness direction retardation Rth5 of the first optical film 40 anda thickness direction retardation Rth6 of the second optical film 41were 110 nm, 220 nm and 300 nm at the retardations Δnd of the liquidcrystal cell of 320 nm, 600 nm and 765 nm, respectively, as in the caseof the simulation shown in FIGS. 7A to 7C. An in-plane retardation Re5of the first optical film 40 and an in-plane retardation Re6 of thesecond optical film 41 were set equal to each other, and a transmittancewas obtained by changing the in-plane retardation in the range of 30 nmto 140 nm.

As the azimuth of the slow axis 40 s is set to 0° (180°) and 90°, thetransmittance becomes independent on the in-plane retardations Re5 andRe6. The simulation results suggest that the in-plane retardations ofthe first optical film 40 and second optical film 41 are cancelled outand only the thickness direction retardations are workable. Namely, itcan be considered that a combination of the first optical film 40 andsecond optical film 41 has the same function as that of the C plate.

When the azimuth of the slow axis 40 s of the first optical film 40 isin a range between 90° and 180°, i.e., when an angle between theabsorption axis 32 a of the front side polarizer 30 and slow axis 40 sis 45° or smaller, a transmittance becomes relatively low. In order toobtain a high contrast, it is therefore preferable to set the anglebetween the absorption axis 32 a of the front side polarizer 30 and slowaxis 40 s to 45° or smaller. This preferable condition is applicablewhen the retardation Δnd of the liquid crystal cell 20 is in a rangebetween 300 nm and 1500 nm, when the in-plane retardation of each of theoptical films 40 and 41 is 300 nm or smaller and when the thicknessdirection retardation is in a range between 50 nm and 300 nm.

When the in-plane retardations Re5 and Re6 of the first optical film 40and second optical film 41 are in a range between 30 nm and 70 nm, thereis a tendency that a transmittance lowers in a range between 90° and180° (i.e., when the angle between the absorption axis 32 a and slowaxis 40 s of the front side polarizer 30 is 45° or smaller) of theazimuth of the slow axis 40 s of the first optical film 40. Thetransmittance becomes lowest particularly when the azimuth of the slowaxis 40 s is near at 135°. It has been confirmed that theabove-described preferred condition is applicable when the thicknessdirection retardations Rth5 and Rth6 of the optical films 40 and 41 arein a range between 110 nm and 300 nm.

When the in-plane retardations Re5 and Re6 of the first optical film 40and second optical film 41 are in a range between 100 nm and 140 nm, thetransmittance takes a maximum value near at 135° with respect to theazimuth of the slow axis 40 s and takes a minimum value in a rangebetween 90° and 135° or in a range between 135° and 180° with respect tothe azimuth of the slow axis 40 s. There is a tendency that atransmittance lowers in a range between 90° and 120° or in a rangebetween 150° and 180° with respect to the azimuth of the slow axis 40 sof the first optical film 40 (i.e., when the angle between theabsorption axis 32 a of the front side polarizer 30 and slow axis 40 sis 15° or larger and 45° or smaller). A still lower transmittance can beobtained when the azimuth of the slow axis 40 s is set in a rangebetween 160° and 175°. It has been confirmed that the above-describedpreferred condition is applicable when the thickness directionretardations Rth5 and Rth6 of the optical films 40 and 41 are in a rangebetween 110 nm and 300 nm. The above-described preferred condition isapplicable also in a range between 140 nm and 300 nm with respect to thein-plane retardations Re5 and Re6 of the optical films 40 and 41.

Generally, in a case where a liquid crystal display unit is observedalong a visual line inclined toward the azimuth parallel to theabsorption axis of a polarizer, better viewing angle characteristics canbe obtained than a case where the liquid crystal display unit isobserved along a visual line inclined toward the azimuth that has 45°relative to the absorption axis (to the right and left side in the firstembodiment). Under the preferred condition described with reference toFIGS. 8A to 8C, it can be predicted that good viewing anglecharacteristics obtained when inclining the visual line to the azimuthparallel to the absorption axis of a polarizer, can be obtained evenwhen inclining the visual line to the right/left direction.

FIGS. 7A to 8C show the case where the optical films 40 and 41 have thesame optical characteristics. Next, description will be made on a casewhere in-plane retardations of the optical films are different from eachother.

FIG. 9 shows simulation results of viewing angle dependency of atransmittance when the in-plane retardation Re5 of the first opticalfilm 40 is changed in a range between 30 nm and 80 nm. The abscissarepresents a viewing angle in the unit of “degree)(°)” along which theliquid crystal display unit is observed, and the ordinate represents atransmittance in the unit of “%”. A retardation Δnd of the liquidcrystal cell 20 is set to 600 nm, and thickness direction retardationsRth5 and Rth6 of the first optical film 40 and second optical film 41are both set to 220 nm. An in-plane retardation Re6 of the secondoptical film 41 is set to 50 nm. An azimuth of the slow axis 40 s of thefirst optical film 40 is set to 135° and an azimuth of the slow axis 41s of the second optical film is set to 45°, namely the arrangement shownin FIG. 6C being adopted.

When there is a large difference between the in-plane retardation Re5 ofthe first optical film 40 and the in-plane retardation Re6 of the secondoptical film 41, there appears a rise in the transmittance starting froma relatively shallow viewing angle of about 10° to 20°. When thein-plane retardation Re5 of the first optical film 40 is equal to thein-plane retardation Re6 of the second optical film 41, thetransmittance starts rising at a viewing angle of 20°. However, a risingdegree is small and the transmittance starts lowering as the viewingangle exceeds 30°. When the in-plane retardation Re5 of the firstoptical film 40 is 60 nm, the transmittance will not rise until theviewing angle reaches 40°.

Degradation of the viewing angle characteristics is small when adifference is 10 nm or smaller between each of the in-plane retardationsRe5 and Re6 of the optical films 40 and 41 and an average of thein-plane retardations of the two optical films 40 and 41.

In the above-described first embodiment, although the negative biaxialfilm is used as the first and second optical films 40 and 41, a positiveA plate may be used instead of the negative biaxial film.

From the viewpoint of compensating a positive thickness directionretardation of the liquid crystal cell 20 in a black state, it ispreferable that a total of the thickness direction retardation Rth5 ofthe first optical film 40 and the thickness direction retardation Rth6of the second optical film 41 falls in a range between 0.5 times and 1time as large as the retardation Δnd of the liquid crystal cell 20(between half of the retardation Δnd the retardation Δnd).

When being applied to an actual liquid crystal display unit, the firstoptical film 40 and second optical film 41 having the same opticalanisotropy are preferably adopted from the viewpoint of reducing thenumber of types of components.

Liquid crystal display units having the structure shown in FIG. 5 aremanufactured and the viewing angle characteristics are evaluated inorder to confirm validity of the simulation results.

A first sample is a liquid crystal display unit in which a retardationΔnd of the liquid crystal cell 20 is 608 nm, in-plane retardations Re5and Re6 of the first optical film 40 and second optical film 41 are both50 nm, and thickness direction retardations Rth5 and Rth6 are both 220nm. An azimuth of the slow axis 40 s of the first optical film 40 is setto 135°, and an azimuth of the slow axis 41 s of the second optical film41 is set to 45°. This sample corresponds to a sample represented by thecurve c in FIGS. 7B and 7C.

When voltage is not applied and a visual line is inclined toward theright/left direction, a good black state hardly having optical leak isable to be obtained up to a viewing angle of 50°. As the viewing angleis increased, optical leak is observed and a tendency similar to thesimulation results is observed.

A second sample is a liquid crystal display unit in which a retardationΔnd of the liquid crystal cell 20 is 430 nm, in-plane retardations Re5and Re6 of the first optical film 40 and second optical film 41 are both120 nm, and thickness direction retardations Rth5 and Rth6 are both 160nm.

An azimuth of the slow axis 40 s of the first optical film 40 is set to90°, and an azimuth of the slow axis 41 s of the second optical film 41is set to 0°. In this case, a good black state hardly having opticalleak is able to be obtained up to a viewing angle of 25° toward theright/left direction. The liquid crystal display unit is observed alonga visual line at a viewing angle of 50° and at azimuth angles of 0° and180° (i.e., right/left azimuths), while the perpendicular conditions ofthe slow axes of the two optical films 40 and 41 were maintained and thefirst optical film 40 and second optical film 41 were rotated. Aphenomenon that the transmittance becomes lowest has been confirmed whenthe azimuth of the slow axis 40 s is set to 160° and the azimuth of theslow axis 41 s is set to 70°. This phenomenon corresponds to the casewherein a minimum value is obtained at an azimuth of the slow axis 40 snear at 160° to 170° when the in-plane retardations Re5 and Re6 are 120nm in FIGS. 8A to 8C.

Second Embodiment

FIG. 10 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to the second embodiment. In the firstembodiment, although two optical films are disposed between the frontside polarizer 30 and liquid crystal cell 20, in the second embodiment,three negative biaxial films are disposed, i.e., a first optical film40, a second optical film 41 and a third optical film 42. Otherstructures are the same as those of the first embodiment.

FIG. 11 shows simulation results of the viewing angle characteristics ofthe liquid crystal display device of the second embodiment. The abscissarepresents a viewing angle in the unit of “degree)(°)”, and the ordinaterepresents a transmittance in the unit of “%”. A retardation Δnd of theliquid crystal cell 20 is set to 900 nm, in-plane retardations Re7 toRe9 of the first to third optical films 40 to 42 are all set to 50 nm,and thickness direction retardations Rth7 to Rth9 are all set to 220 nm.An azimuth φ7 of a slow axis 40 s of the first optical film 40 is set to45°. A viewing angle dependency of a transmittance is calculated forvarious combinations of an azimuth φ8 of a slow axis 41 s of the secondoptical film 41 and an azimuth φ9 of a slow axis 42 s of the thirdoptical film 42.

It has been found that good viewing angle characteristics are obtainedby setting the azimuth φ8 of the slow axis 41 s of the second opticalfilm 41 to 0° and the azimuth φ9 of the slow axis 42 s of the thirdoptical film 42 to 90°.

A transmittance is obtained when the azimuth φ8 of the slow axis 42 s ofthe third optical film 42 is changed, while maintaining theperpendicular conditions between the slow axis 41 s of the secondoptical film 41 and the slow axis 42 s of the third optical film 42.

FIG. 12 shows the results. The abscissa represents the azimuth φ8 of aslow axis 41 s of the second optical film 41 in the unit of“degree)(°)”, and the ordinate represents a transmittance in the unit of“%”. An azimuth of a visual line is set to 0°, and a viewing angle isset to 50°. It can be understood that as the azimuth φ8 of the slow axis41 s is set in a range between 90° and 180°, a transmittance becomeslower than in a range between 0° and 90°.

It is difficult to adhere two optical films precisely so as to make theslow axes cross perpendicularly, and the slow axes sometimes shift fromthe perpendicular relation. In the following, consideration will be madeon the influence of a shift from the perpendicular relation between theslow axis 41 s of the second optical film 41 and the slow axis 42 s ofthe third optical film 42.

When the slow axis 41 s of the second optical film 41 and the slow axis42 s of the third optical film 42 are disposed parallel or perpendicularto the absorption axes 32 a and 11 a of the front side and rear sidepolarizers, optical anisotropy of the second and third optical films 41and 42 is hard to be observed. Therefore, even if the slow axis 41 s andslow axis 42 s shift from the perpendicular relation, its influence issmall. For example, a transmittance will not vary remarkably whenobserving squarely.

However, when the slow axis 41 s and slow axis 42 s are disposed at anangle of 45° relative to the absorption axes 32 a and 11 a of thepolarizers, optical anisotropy of the second and third optical films 41and 42 is observed remarkably. Therefore, if the slow axis 41 s and slowaxis 42 s shift from the perpendicular relation even by 0.5°, atransmittance when observing squarely rises greatly, resulting in alowered contrast.

It is therefore preferable to dispose the slow axis 41 s of the secondoptical film 41 and the slow axis 42 s of the third optical film 42parallel or perpendicular to the absorption axes 32 a and 11 a of thefront side and rear side polarizers.

Similar simulation is conducted by setting the azimuth φ7 of the slowaxis 40 s of the first optical film 40 to 135° and changing the azimuthsof the slow axis 41 s of the second optical film 41 and the slow axis 42s of the third optical film 42. It has been found from the simulationresults that it is favorable that the azimuth φ7 of the slow axis 40 sof the first optical film 40 is set to 45°. It can be understood fromthis that it is more preferable to dispose the absorption axis 32 a ofthe front side polarizer 30 and the slow axis 40 s of the first opticalfilm 40 at an angle therebetween that is in a range between 45° and135°, than to dispose both the axes in parallel. This preferablecondition is applicable when the retardation Δnd of the liquid crystalcell 20 is in a range between 550 nm and 1500 nm, when the in-planeretardation of each of the optical films 40 to 42 is in a range between30 nm and 300 nm and when the thickness direction retardation is in arange between 50 nm and 300 nm.

Simulation of the viewing angle characteristics is conducted by changinga combination of the in-plane retardations Re7 to Re9 of the first tothird optical films 40 to 42. It has been found from the simulationresults that it is preferable to set all the in-plane retardations Re7to Re9 equal to each other. It has also been found that the viewingangle characteristics not inferior to the case wherein all the in-planeretardations Re7 to Re9 are equal, can be obtained when there is adifference not larger than 10 nm between each of the in-planeretardations Re7 to Re9 and an average of the retardations Re7 to Re9.

It is preferable that the in-plane retardation Re7 of the first opticalfilm 40 disposed nearest to the front side polarizer 30 is set to 30 nmto 80 nm. Regarding the thickness direction retardations, although it isnot necessary for the optical films 40 to 42 to have the same value, itis preferable that a total of the thickness direction retardations ofthe three optical films 40 to 42, i.e., Rth7+Rth8+Rth9, falls in a rangebetween 0.5 times and 1 time the retardation Δnd of the liquid crystalcell 20 (between half of the retardation Δnd the retardation Δnd).

A liquid crystal display unit is manufactured in which a retardation Δndof the liquid crystal cell is about 900 nm, in-plane retardations Re7 toRe9 of the three optical films 40 to 42 are all 50 nm, thicknessdirection retardations Rth7 to Rth9 are all 220 nm, and azimuths φ7 toφ9 of the slow axes are 45°, 135° and 45°, respectively. The viewingangle characteristics of the liquid crystal display unit are actuallymeasured and it is confirmed that the above-described simulation resultsare almost satisfied.

Third Embodiment

FIG. 13 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to the third embodiment. In the secondembodiment, although three optical films 40 to 42 are disposed betweenthe front side polarizer 30 and liquid crystal cell 20, in the thirdembodiment, a fourth optical film 43 having negative biaxial anisotropyis disposed between the first optical film 40 and front side polarizer30. Four optical films 40 to 43 in total are therefore disposed.

A retardation Δnd of the liquid crystal cell is set to 1180 nm, in-planeretardations Re7 to Re10 of the first to fourth optical films 40 to 43are all set to 50 nm, and thickness direction retardations Rth7 to Rth10are all set to 220 nm. Simulation of the viewing angle characteristicsis conducted for various combinations of azimuths of in-plane slow axesof the optical films 40 to 43. It has been found from the simulationresults that the best viewing angle characteristics can be obtained bymaking the in-plane slow axes of adjacent two optical filmsperpendicular to each other and disposing the in-plane slow axes 43 s ofthe fourth optical film 43 nearest to the front side polarizer 30 to beparallel to the absorption axis 32 a of the front side polarizer 30. Ithas also been found that relatively good viewing angle characteristicscan be obtained by setting an angle between the in-plane slow axis 43 sof the fourth optical film 43 and the absorption axis 32 a of the frontside polarizer 30 to 45° or smaller than 45°.

If a negative biaxial film is further laminated, the retardation Δnd ofthe liquid crystal layer can be compensated. However, in order tomaintain a good display state when voltage is applied, it is preferableto set the retardation Δnd of the liquid crystal layer to 1500 nm orsmaller than 1500 nm.

Next, a modification of the third embodiment will be described. In thethird embodiment, although four optical films 40 to 43 are disposedbetween the liquid crystal cell 20 and front side polarizer 30 as shownin FIG. 13, in the modification, five optical films are disposed. Aretardation Δnd of the liquid crystal layer is set to 1480 nm, and thein-plane retardation and thickness direction retardation of each opticalfilm are set to be the same as the third embodiment.

Simulation of the viewing angle characteristics is conducted for variouscombinations of azimuths of in-plane slow axes of the optical films. Ithas been found from the simulation results that the best viewing anglecharacteristics can be obtained by making the in-plane slow axes ofadjacent two optical films perpendicular to each other and disposing thein-plane slow axes of the optical film nearest to the front sidepolarizer 30 to be perpendicular to the absorption axis 32 a of thefront side polarizer 30. It has also been found that relatively goodviewing angle characteristics can be obtained when an angle between thein-plane slow axis of the optical film nearest to the front sidepolarizer 30 and the absorption axis 32 a of the front side polarizer 30is in a range between 45° and 135°.

It can be understood from the above-described simulation results that ifthe number of positive A plates or negative biaxial films disposedbetween the liquid crystal cell and front side polarizer is even, it ispreferable to set an angle between the in-plane slow axis of the opticalfilm nearest to the front side polarizer and the absorption axis of thefront side polarizer in a range between −45° to 45°, or more preferableto set the angle to 0°. It can also be understood that if the number isodd and larger than 3, it is preferable to set an angle between thein-plane slow axis of the optical film nearest to the front sidepolarizer and the absorption axis of the front side polarizer in a rangebetween 45° and 135°, or more preferable to set the angle to 90°. Ineither case, it is preferable to dispose the in-plane slow axes ofadjacent optical films to be perpendicular to each other.

Fourth Embodiment

FIG. 14 is a schematic diagram showing the outline structure of a liquidcrystal display unit according to the fourth embodiment. A first opticalfilm 40 and a second optical film 41 are disposed between a front sidepolarizer 30 and liquid crystal cell 20. The first optical film 40 isdisposed on the side of the front side polarizer 30, and the secondoptical film 41 is disposed on the side of the liquid crystal cell 20. Athird optical film 42 is disposed between a rear side polarizer 10 andliquid crystal cell 20. The structures of the rear side polarizer 10,liquid crystal cell 20 and front side polarizer 30 are the same as thoseof the liquid crystal display unit shown in FIG. 1.

A retardation Δnd of the liquid crystal cell 20 is, for example, 410 nm.A negative biaxial film or a positive A plate is used as the first tothird optical films 40 to 42. Namely, the first to third optical films40 to 42 have optical anisotropy satisfying nx>ny≧nz. In-planeretardations Re5 to Re7 of the first to third optical films 40 to 42are, for example, 50 nm, and thickness direction retardations Rth5 toRth7 are, for example, 90 nm.

An azimuth of an absorption axis 11 a of the rear side polarizer 10 isset to 45°, and an azimuth of an absorption axis 32 a of the front sidepolarizer 30 is set to 135°. An in-plane slow axis 41 s of the secondoptical film 41 and an in-plane slow axis 42 s of the third optical film42 are disposed to be perpendicular to each other. A transmittance isobtained by simulation by setting an azimuth angle φ5 of a slow axis 40s of the first optical film 40 to 45° and 135° and changing an in-planeslow axis azimuth φ6 of the second optical film in a range between 0°and 180°.

FIG. 15 shows simulation results. The abscissa represents an in-planeslow axis direction φ6 in the unit of “degree)(°)”, and the ordinaterepresents a transmittance in the unit of “%”. The transmittance isobtained through observation under the conditions of an azimuth of 0°and a viewing angle of 50°. The “viewing angle” is defined as an anglebetween a normal direction of a liquid crystal cell and a visual line.

There is a tendency that a transmittance lowers in a range between 10°and 20° and in a range between 80° and 90° with respect to the in-planeslow axis azimuth φ6, at the in-plane slow axis azimuth φ5 of 45°. Onthe other hand, there is a tendency that a transmittance lowers in arange between 30° and 60° with respect to the in-plane slow axis azimuthφ6 at the in-plane slow axis azimuth φ5 of 135°. The simulation resultsshown in FIG. 15 are obtained through observation at a visual lineinclined toward the azimuth of 0°. The all-around viewing anglecharacteristics are calculated next.

Simulation results of the all-around viewing angle characteristics areshown in FIGS. 16A to 16D. FIGS. 16A to 16D show equi-transmittancecurves. A center indicates a transmittance at a viewing angle of 0°,i.e., when facing squarely, and four concentric circles correspond tothe transmittances at viewing angles of 20°, 40°, 60° and 80°sequentially from the innermost circle toward the outer circle. Right,upper, left and lower directions correspond to azimuths of 0°, 90°, 180°and 270°, respectively. Transmittances indicated by theequi-transmittance curves are 0.02%, 0.05%, 0.1%, 0.2%, 0.5% and 1%sequentially from the innermost curve toward the outer curve.

FIG. 16A shows the viewing angle characteristics at in-plane slow axisazimuths of φ5=45° and φ6=90°, FIG. 16B shows the viewing anglecharacteristics at in-plane slow axis azimuths of φ5=135° and φ6=30°,FIG. 16C shows the viewing angle characteristics at in-plane slow axisazimuths of φ5=135° and φ6=45°, and FIG. 16D shows the viewing anglecharacteristics at in-plane slow axis azimuths of φ5=135° and φ6=60°.

Although the viewing angle characteristics in the upper, lower, rightand left azimuths are relatively good at the in-plane slow axis azimuthsφ5 and φ6 of 45° and 90, respectively, an increase in the transmittanceis remarkable when the visual line is inclined toward the azimuths of45° and 135°. Simulation is conducted for the other conditions that atransmittance lowers at the in-plane slow axis azimuth φ5 of 45°, i.e.,for the conditions of the in-plane slow axis azimuth φ6 of 15°, andsimilar to the case of the in-plane slow axis azimuth φ6 of 90°, it hasbeen found that an increase in the transmittance is remarkable when thevisual line is inclined toward the azimuths of 45° and 135°. Incontrast, as shown in FIGS. 16B to 16D, at the in-plane slow axisazimuth φ5 of 135°, an increasing tendency of the transmittance when thevisual line is inclined toward the azimuths of 45° and 135° is small.Particularly at the in-plane slow axis azimuth φ6 of 45°, it has beenfound that an increase in the transmittance, when the visual line isinclined toward the azimuths of 45° and 135°, is extremely small.

FIG. 17 shows simulation results of transmittances when the in-planeretardations Re5 to Re7 of the first to third optical films 40 to 42 arechanged under the conditions in which these in-plane retardations Re5 toRe7 are equal to one another. Simulation is conducted at each of thein-plane retardations Re5 to Re7 of 30 nm, 50 nm, 70 nm, 100 nm and 120nm. The in-plane slow axis azimuth φ5 is set to 135°, and the in-planeslow axis 41 s of the second optical film 41 and the in-plane slow axis42 s of the third optical film 42 are perpendicular to each other. Theabscissa of FIG. 17 represents an in-plane slow axis azimuth φ6 in theunit of “degree) (°)”, and the ordinate represents a transmittance inthe unit of “%”. The transmittance is observed along a visual line of anazimuth of 0° and a viewing angle of 50°.

When the in-plane retardations Re5 to Re7 are smaller than 50 nm, thereis a tendency that the transmittance takes a minimum value near at anin-plane slow axis azimuth φ6 of 45°. On the other hand, when thein-plane retardations Re5 to Re7 are larger than 50 nm, thetransmittance takes a minimum value near at an in-plane slow axisazimuth φ6 of 30° and 60° and a maximum value near at an in-plane slowaxis azimuth φ6 of 45°. As the in-plane retardations Re5 to Re7 becomelarger than 50 nm, the transmittance increases with this tendency beingmaintained. As seen from these evaluation results, it is preferable toset the in-plane retardations Re5 to Re7 in a range between 30 nm and 70nm in order to obtain relatively good viewing angle characteristics.

FIG. 18 shows simulation results of viewing angle characteristics whenthe in-plane retardation Re5 of the first optical film 40 is differentfrom the in-plane retardations Re6 and Re7 of the second and thirdoptical films 41 and 42. The abscissa represents a slow axis azimuth φ6of the second optical film 41 in the unit of “degree)(°)”, and theordinate represents a transmittance in the unit of “%”. Thetransmittance is observed along a visual line of an azimuth of 0° and aviewing angle of 50°. The in-plane retardations Re6 and Re7 of thesecond and third optical films 41 and 42 are both set to 50 nm.

It can be seen that the transmittances change almost similarly at leastin a range between 30 nm and 70 nm with respect to the in-planeretardation Re5. A low transmittance is obtained in a range between 30°and 60° with respect to the in-plane slow axis azimuth φ6 of the secondoptical film 41. These evaluation results indicate that the viewingangle characteristics are not influenced greatly even if the in-planeretardation Re5 of the first optical film 40 is not uniform in thein-plane and varies in a range between 30 nm and 70 nm.

FIG. 19 shows simulation results of viewing angle characteristics whenthe in-plane retardation Re7 of the third optical film 42 is differentfrom the in-plane retardations Re5 and Re6 of the first and secondoptical films 40 and 41. The abscissa represents a slow axis azimuth φ6of the second optical film 41 in the unit of “degree)(°)”, and theordinate represents a transmittance in the unit of “%”. Thetransmittance is observed along a visual line of an azimuth of 0° and aviewing angle of 50°. The in-plane retardations Re5 and Re6 of the firstand second optical films 40 and 41 were both set to 50 nm.

The transmittances change almost similarly at least in a range betweennm and 70 nm with respect to the in-plane retardation Re7. A lowtransmittance is obtained in a range between 30° and 60° with respect tothe in-plane slow axis azimuth φ6 of the second optical film 41. Theseevaluation results indicate that the viewing angle characteristics arenot influenced greatly even if the in-plane retardation Re7 of the thirdoptical film 42 is not uniform in the in-plane and varies in a rangebetween 30 nm and 70 nm.

FIG. 20 shows simulation results of viewing angle characteristics whenthe in-plane retardation Re6 of the second optical film 41 is differentfrom the in-plane retardations Re5 and Re7 of the first and thirdoptical films 40 and 42. The abscissa represents a slow axis azimuth φ6of the second optical film 41 in the unit of “degree)(°)”, and theordinate represents a transmittance in the unit of “%”. Thetransmittance is observed along a visual line of an azimuth of 0° and aviewing angle of 50°. The in-plane retardations Re5 and Re7 of the firstand third optical films 40 and 42 are both set to 50 nm.

The transmittances change almost similarly at least in a range between30 nm and 70 nm with respect to the in-plane retardation Re6. A lowtransmittance is obtained in a range between 30° and 60° with respect tothe in-plane slow axis azimuth φ6 of the second optical film 41. Theseevaluation results indicate that the viewing angle characteristics arenot influenced greatly even if the in-plane retardation Re6 of thesecond optical film 41 is not uniform in the in-plane and varies in arange between 30 nm and 70 nm.

It can be considered from the above-described analysis results that itis preferable to set the in-plane slow axis azimuth φ5 of the firstoptical film 40 to 135° in order to realize good viewing anglecharacteristics. Namely, it is preferable that the in-plane slow axis 40s of the first optical film 40 and the absorption axis 32 a of the frontside polarizer 30 are disposed to be parallel to each other. In thiscase, the in-plane slow axis azimuth φ6 of the second optical film 41 ispreferably set in a range between 30° and 60°. Namely, it is preferablethat an angle between the in-plane slow axis 41 s of the second opticalfilm 41 and the absorption axis 32 a of the front side polarizer 30 isset in a range between 75° and 105°. It is preferable that the in-planeslow axis 41 s of the second optical film 41 and the in-plane slow axis42 s of the third optical film 42 are disposed to be perpendicular toeach other. The in-plane axis retardations Re5 to Re7 of the first tothird optical films 40 to 42 are preferably set in a range between 30 nmand 70 nm. Further, it is preferable that a difference between each ofthe in-plane retardations Re5 to Re7 of the first to third optical films40 to 42 and an average of the in-plane retardations of the first tothird optical films 40 to 42 is 10 nm or smaller than 10 nm.

The above-described structures can effectively suppress the viewingangle characteristics from being degraded when the retardation Δnd ofthe liquid crystal cell 20 is in a range between 550 nm and 1080 nm.

It is preferable that a total of the in-plane retardations Rth5 to Rth7of the first to third optical films 40 to 42 falls in a range between0.5 times and 1 time the retardation Δnd of the liquid crystal cell 20(between half of the retardation Δnd and the retardation Δnd).

Further, the viewing angle improvement effects can be obtained even ifthe first to third optical films having an in-plane retardation largerthan 0 and not larger than 300 nm are used. A negative biaxial filmhaving such optical anisotropy is available easily.

Fifth Embodiment

Next, the fifth embodiment will be described. For the liquid crystaldisplay device of the fifth embodiment, the thickness directionretardations Rth5 to Rth7 of the first to third optical films 40 to 42of the liquid crystal display device shown in FIG. 14 are set to 300 nm.The thickness direction retardation of 300 nm corresponds to a maximumvalue in a range capable of uniformly working an optical film usingnorbornene based cyclic olefin. A retardation Δnd of the liquid crystalcell 20 is set to 1080 nm and a pretilt angle of liquid crystalmolecules is set to 89.9°. An in-plane slow axis azimuth φ5 of the firstoptical film 40 is set to 135°, and the in-plane slow axis 42 s of thethird optical film 42 and the in-plane slow axis 41 s of the secondoptical film 41 are disposed to be perpendicular to each other.Transmittance simulation is conducted in three cases where all thein-plane retardations Re5 to Re7 of the first to third optical films 40to 42 are set to 30 nm, 50 nm, and 70 nm.

FIG. 21 shows simulation results. The abscissa represents a slow axisazimuth φ6 of the second optical film 41 in the unit of “degree)(°)”,and the ordinate represents a transmittance in the unit of “%”. Thetransmittance is observed along a visual line of an azimuth of 0° and aviewing angle of 50°. A transmittance becomes low in a range between 15°and 70° with respect to the in-plane slow axis azimuth φ6 in all casesof the in-plane retardations Re5 to Re7 of the first to third opticalfilms 40 to 42 of 30 nm, 50 nm, and 70 nm. This range includes thepreferable range between 30° and 60° with respect to the in-plane slowaxis azimuth φ6 of the fourth embodiment.

It is expected from the fourth and fifth embodiments that the viewingangle improvement effects can be obtained even if the first to thirdoptical films 40 to 42 are used having the thickness directionretardation being in a range between 50 nm and 300 nm.

Sixth Embodiment

FIG. 22 is a schematic diagram showing the outline structure of a liquidcrystal display device according to the sixth embodiment. In thefollowing, description will be made on different points from the liquidcrystal display device of the fourth embodiment shown in FIG. 14. Afourth optical film 43 is disposed between the second optical film 41and liquid crystal cell 20. Other arrangements are the same as those ofthe fourth embodiment.

The first to third optical films 40 to 42 are negative biaxial films.Each of the in-plane retardations Re5 to Re 7 is 50 nm, and each of thethickness direction retardations Rth5 to Rth7 is 220 nm. The negativebiaxial film having such optical anisotropy can be realized by usingnorbornene based cyclic olefin, and is readily available. The fourthoptical film 43 is a negative C plate having a thickness directionretardation Rth8 of 220 nm. Namely, the in-plane retardation of thefourth optical film 43 is 0.

FIG. 23 shows simulation results of a relation between a retardation Δndof the liquid crystal cell 20 and a transmittance. The abscissarepresents a retardation Δnd of the liquid crystal cell 20 in the unitof “nm”, and the ordinate represents a transmittance in the unit of “%”.The in-plane slow axis azimuth φ5 of the first optical film 40 and thein-plane slow axis azimuth φ7 of the third optical film 42 are both setto 135°, and the in-plane slow axis azimuth φ6 of the second opticalfilm 41 is set to 45°. The transmittance is observed along a visual lineof an azimuth of 0° and a viewing angle of 50°.

A curve b in FIG. 23 indicates a transmittance of the liquid crystaldisplay device of the sixth embodiment shown in FIG. 22, and a curve aindicates a transmittance of the liquid crystal display device of thefourth embodiment shown in FIG. 14. Both the transmittances have adownward convex curve and have similar shapes. The liquid crystaldisplay device of the sixth embodiment shown in FIG. 22 has a minimumtransmittance at a larger retardation Δnd. A difference betweenretardations Δnd indicating minimum transmittances corresponds to thethickness direction retardation Rth8 of the fourth optical film 43 madeof the negative C plate. As the fourth optical film 43 of the negative Cplate is disposed, good viewing angle characteristics can be realizedfor the liquid crystal cell 20 having a larger retardation Δnd.

An optical film available in markets as a negative C plate does notalways have an in-plane retardation of strictly 0, but has an in-planeretardation of about 7 nm in some cases. As the fourth optical film 43,a negative biaxial film having an in-plane retardation of 7 nm orsmaller than 7 nm may be used.

Although the fourth optical film 43 is made of a single negative C platein the sixth embodiment, it may be made of a plurality of negative Cplates. Further, the fourth optical film 43 may be disposed between theliquid crystal cell 20 and third optical film 42. Furthermore, when aplurality of negative C plates are used, some C plates may be disposedbetween the liquid crystal cell 20 and second optical film 41, and theremaining C plates are disposed between the liquid crystal cell 20 andthird optical film 42.

As described above, by disposing the fourth optical film 43, aninsufficient total of the thickness direction retardations Rth5 to Rth7of the first to third optical films 40 to 42 is compensated so thatbetter viewing angle characteristics can be obtained.

The thickness direction retardation of the fourth optical film 43 ispreferably in a range between 50 nm and 300 nm. The negative C platehaving the thickness direction retardation in this range is readilyavailable. Providing the fourth optical film 43 is effectiveparticularly when the retardation Δnd of the liquid crystal cell 20 isin a range between 460 nm and 1380 nm.

It is preferable that a total of the thickness direction retardationsRth5 to Rth8 of the first to fourth optical films 40 to 43 falls in arange between 0.5 times and 1 time the retardation Δnd of the liquidcrystal cell 20 (between half of the retardation Δnd the retardationΔnd).

Seventh Embodiment

FIG. 24 is a schematic diagram showing the outline structure of a liquidcrystal display device according to the seventh embodiment. In theseventh embodiment, a fifth optical film 44 and a sixth optical film 45are disposed instead of the fourth optical film 43 of the liquid crystaldisplay device of the sixth embodiment shown in FIG. 22. The fifthoptical film 45 is disposed on the side of the second optical film 41,whereas the sixth optical film 45 is disposed on the side of the liquidcrystal cell 20. The arrangement of other optical films is the same asthe sixth embodiment. The fifth optical film 44 and sixth optical film45 are positive A plates or negative biaxial films. The in-plane slowaxis azimuth 44 s of the fifth optical film 44 and the in-plane slowaxis azimuth 45 s of the sixth optical film 45 are preferably disposedto be perpendicular to each other.

A relation between a transmittance and a retardation Δnd of the liquidcrystal cell 20 is obtained through simulation, under the conditionsthat each of the in-plane retardations Re5 to Re7, Re9 and Re10 of thefirst to third optical films 40 to 42, and fifth and sixth optical films44 and 45 is 50 nm, and each of the thickness direction retardationsRth5 to Rth7, Rth9 and Rth10 is 220 nm. The transmittance is observedalong a visual line of an azimuth of 0° and a viewing angle of 50°.

The simulation result is indicated by a curve c in FIG. 23. The in-planeslow axis azimuth φ5 of the first optical film 40 and the in-plane slowaxis azimuth φ7 of the third optical film 42 are set to 135°, and thein-plane slow axis azimuth φ6 of the second optical film 41 is set to45°. The in-plane slow axis azimuth φ9 of the fifth optical film 44 isset to 90°, and the in-plane slow axis azimuth φ10 of the sixth opticalfilm 45 is set to 10°.

A shape of the curve c is similar to the shapes of other curves a and b.As compared to the curve b indicating the transmittance of the liquidcrystal display device shown in FIG. 22, the curve c takes a minimumvalue at a larger retardation Δnd. This is because a total of thethickness direction retardations Rth9 and Rth10 of the fifth and sixthoptical films 44 and 45 is larger than the thickness directionretardation Rth8 of the fourth optical film 43 shown in FIG. 22. In thismanner, the viewing angle compensation can be performed even in the casethat the retardation Δnd of the liquid crystal cell is larger, bydisposing the two positive A plates or negative biaxial films havingin-plane slow axes, which are perpendicular to each other, between theliquid crystal cell 20 and second optical film 41.

A relation between a transmittance and an in-plane slow axis azimuth φ9of the fifth optical film 44 is obtained through simulation. Thein-plane slow axis azimuth φ9 is varied while maintaining theperpendicular relation between the in-plane slow axis 44 s of the fifthoptical film 44 and the in-plane slow axis 45 s of the sixth opticalfilm 45.

FIG. 25 shows simulation results. The abscissa represents an in-planeslow axis azimuth φ9 in the unit of “degree (°)”, and the ordinaterepresents a transmittance in the unit of “%”. The transmittance isobserved along a visual line of an azimuth of 0° and a viewing angle of50°. A relatively low transmittance is obtained in a range between 90°and 180° with respect to the in-plane slow axis azimuth φ9 of the fifthoptical film 44. In order to obtain good viewing angle characteristics,the in-plane slow axis azimuth φ9 is set preferably in a range between90° and 180°, and more preferably in a range between 115° and 155°.Namely, it is preferable that the in-plane slow axis 44 s of the fifthoptical film 44 and the in-plane slow axis 41 s of the adjacent secondoptical film 41 is made perpendicular to each other or a shift anglefrom the perpendicular relation is set in a range of ±20°.

It is preferable that a positive A plate or negative biaxial film havingan in-plane retardation larger than 30 nm and smaller than or equal to70 nm and a thickness direction retardation larger than or equal to 50nm and smaller than or equal to 300 nm is used as the first to thirdoptical films 40 to 42 and fifth and sixth optical films 44 and 45. Thestructure of the seventh embodiment is effective particularly in a rangebetween 510 nm and 1380 nm with respect to the retardation Δnd of theliquid crystal cell 20.

In the seventh embodiment, a pair of fifth and sixth optical films 44and 45 having in-plane slow axes perpendicular to each other is disposedbetween the second optical film 41 and liquid crystal cell 20. Instead,a plurality of pairs of negative biaxial films having in-plane slow axesperpendicular to each other may be disposed. By disposing the pluralityof pairs, viewing angle compensation can be performed even if theretardation Δnd of the liquid crystal cell 20 is larger.

It is preferable that a total of the thickness direction retardationsRth5 to Rth7, Rth9 and Rth10 of the first to third optical films 40 to42 and fifth and sixth optical films 44 and 45 falls in a range between0.5 times and 1 time the retardation Δnd of the liquid crystal cell 20(between half of the retardation Δnd the retardation Δnd).

Eighth Embodiment

FIG. 26 is a schematic diagram showing the outline structure of a liquidcrystal display device according to the eighth embodiment. A firstoptical film 40 and a second optical film 41 made of negative biaxialfilms having the same optical characteristics are disposed in this orderfrom the polarizer toward the liquid crystal cell 20 between the frontside polarizer 30 and liquid crystal cell 20. A third optical film 42and a fourth optical film 43 made of negative biaxial films having thesame optical characteristics are disposed in this order from the liquidcrystal cell 20 toward the rear side polarizer 10 between the rear sidepolarizer 10 and liquid crystal cell 20. The optical characteristics ofthe four optical films 40 to 43 are the same. Two negative biaxial filmsare disposed on both sides of the liquid crystal cell 20. The absorptionaxes 32 a and 11 a of the polarizers are respectively parallel (sameazimuth angle), to the slow axes 40 s and 12 s of the adjacent negativebiaxial films, and the slow axes 41 s and 43 s of the negative biaxialfilms on the side of the liquid crystal cell 20 are respectivelyperpendicular to the slow axes 40 s and 12 s of the negative biaxialfilms on the side of the polarizer.

A retardation Δnd of the liquid crystal cell 20 is, for example, 500 nm.The first to fourth optical films 40 to 43 have optical anisotropysatisfying nx>ny≧nz. Each of the in-plane retardations Re5 to Re8 of thefirst to fourth optical films 40 to 43 satisfies 0 nm<Re≦300 nm andpreferably 30 nm≦Re≦70 nm, e.g., 50 nm, and each of the thicknessdirection retardations Rth5 to Rth8 satisfies 50 nm≦Rth≦300 nm andpreferably 90 nm≦Rth≦300 nm, e.g., 90 nm. A maximum Rth=1200 nm becomespossible by using four optical films.

Slow axis azimuths of a negative biaxial film and opticalcharacteristics of the liquid crystal display device in an off state arestudied through simulation. A retardation Δnd of the liquid crystal cell20 is set to 500 nm, and the four optical films 40 to 43 have the sameoptical characteristics of Re=50 nm and Rth=90 nm. An total of Rth ofall optical films is 360 nm which is about more than 70% of theretardation of 500 nm of the liquid crystal cell. An azimuth of theabsorption axis 11 a of the rear side polarizer 10 is 45°, and anazimuth of the absorption axis 32 a of the front side polarizer 30 is135°. Influence of the slow axis azimuth of each of the four opticalfilms 40 to 43 upon a transmittance in an off state is studied.

FIG. 27 is a graph showing a change in a transmittance (optical leak) inan off state of each sample with respect to a change in an angle betweena substrate normal and a viewing angle to the right and left from thesubstrate normal direction. As shown, a sample S1 has slow axes ofoptical films disposed at 0° and 90° of the center azimuths of theabsorption axes of the polarizers in cross-Nichol configuration, andsamples S2 to S8 of seven types selects slow axis azimuths of fouroptical films from 45° and 135° perpendicular or parallel to theabsorption axes of the polarizers.

The sample S4 has excellent characteristics that a transmittance isextremely low in a wide angle range even if the viewing angle isinclined from the substrate normal direction. The slow axes of theoptical films 40 and 43 on the polarizer side are 135° and 45° which areparallel to the absorption axes of the adjacent polarizers. However,transmittances of the samples S2 sand S3 having the slow axis azimuthsof 135° and 45° of the optical films 40 and 43 increase steeply as theviewing angle becomes large, and are not preferable.

In the four optical films of the sample 4 having the most excellentcharacteristics, the slow axis of the optical film on the polarizer sideis parallel to the absorption axis of the polarizer, and the slow axisof the optical film on the liquid crystal cell side is perpendicular tothe slow axis of the optical film on the polarizer side. It can beconsidered that this slow axis arrangement is most effective forsuppressing optical leak at a viewing angle inclined from the substratenormal direction.

The first to fourth optical films 40 to 43 may be configured of positiveA plates instead of negative biaxial films. Also in this case, thepreferred azimuths of the slow axes are considered to remain unchanged.

There is an error of angle setting. Although the optical films 40-41 andthe optical films 42-43 are prepared in a bonded state, it is consideredthat a relative position to the polarizer is established during assemblyin many cases. Influence of an angle error generated at the assemblystage has been checked. Influence of a shift of the slow axis azimuth ofeach of the optical films 40 and 43 from the absorption axis azimuth ofadjacent polarizers has been studied. The slow axis azimuths of theoptical films 41 and 42 are set perpendicular to the slow axis azimuthsof the adjacent optical films 40 and 43, respectively. An observingazimuth is a right (azimuth of 0°) and viewing angle is 50°.

FIG. 28 is a graph showing a change in a transmittance at a viewingangle of 50° in an absence of voltage application, with respect to anazimuth, when the perpendicular optical film pair 40-41 is rotatedrelative to the absorption axis azimuth (135°) of the polarizer 30 andthe perpendicular optical film pair 42-43 is rotated relative to theabsorption axis azimuth (45°) of the polarizer 10. The abscissarepresents a slow axis azimuth of the optical film 40, and the ordinaterepresents a transmittance. The characteristics of Samples S12, S13 andS14 are particularly excellent in a range between 120° and 150° withrespect to the slow axis azimuth of the optical film 40. The slow axisazimuths between 120° and 150° of the first optical film 40 are in arange of ±15° from the absorption axis azimuth (135°) of the adjacentpolarizer 30. Samples S12, S13 and S14 have the slow axes of the fourthoptical film 43 of 30°, 45° and 60° which are in a range of ±15° fromthe absorption axis azimuth (45°) of the adjacent polarizer 10. It canbe considered from these results that the azimuth range of ±15° can beregarded as approximately the same azimuth. If an angle between twoazimuths has a relation of a range between −15° and +15°, this relationis called approximately parallel, and a relation of a range between 75°and 105° is called approximately perpendicular.

In the above-described studies, the in-plane retardations Re of the fouroptical films are all set to 50 nm. How the relation shown in FIG. 28rotating the slow axes of the first and fourth optical films changes, ischecked at Re of 30 nm and 70 nm.

FIG. 29 shows transmittances at Re=30 nm, and FIG. 30 showstransmittances at Re=70 nm. In FIG. 29, although the transmittance(optical leak) increases slightly, almost a similar tendency to that ofFIG. 28 can be observed. There is no change in the conclusion that theslow axis azimuth of an optical film adjacent to the polarizer ispreferably in a range of ±15° from the absorption axis of the polarizer.

In FIG. 30, transmittances of samples S32, S33 and S34 near at 135°change to an upward convex shape, and are higher than a transmittance ofa sample S35 in a partial portion of a range between 120° and 150°.However, since the transmittances in the range between 120° and 150° arelow, it can be considered that there is no change in the conclusion thatthe slow axis azimuth of an optical film adjacent to the polarizer ispreferably in a range of ±15° from the absorption axis of the polarizer.In-plane retardation Re is preferably 30 nm to 70 nm. If FIG. 30 isexcluded and FIGS. 28 and 29 are adopted, the optical characteristicsare more preferable and Re of 30 nm to 50 nm is more preferable.

In the above-described studies, all the four optical films has the sameoptical characteristics. In actual optical films, the perfectly uniformoptical characteristics, Re and Rth in particular, are difficult torealize. Reviews are conducted through simulation for the case that onlythe second optical film 41 has a different Re from those of the otheroptical films.

FIG. 31 is a graph showing a change in a transmittance (optical leak)with respect to a viewing angle at the in-plane retardations Re of thesecond optical film of 50 nm (S41), 40 nm (S42) and 60 nm (S43). Re ofall other optical films is 50 nm. It can be seen that even if Re of thesecond optical film changes in a range between 40 nm and 60 nm, aremarkable change in the viewing angle characteristics does not appear.Namely, even if Re of the second optical film has a variation of ±10 nmat the center of 50 nm, it can be considered that a significant changein the viewing angle characteristics will not occur. Similar simulationshave been conducted also for the first, third and fourth optical films,and it has been found that influence upon the visual anglecharacteristics is small in each case. It can be considered thatvariation in a range of ±10 nm with respect to in-plane retardation Reof the optical film is allowable. The optical characteristics having asuppressed retardation variation of ±10 nm are called approximately thesame optical characteristics.

A liquid crystal display device is actually manufactured and an exteriorobservation is conducted. A retardation Δnd of a liquid crystal cell isset to about 500 nm, each optical film is made of norbornene based COP,the retardations are set to Re=50 nm and Rth=90 nm. The optical films40, 41, 42 and 43 are bonded at slow axis azimuths of 135°, 45°, 135°and 45°, respectively. It has been confirmed from the exteriorobservation that good background visual angle characteristics areobtained similar to the above-described simulations.

Reviews are conducted through simulation for the case that theretardation Δnd is set to 1500 nm which is considered optimum and thethickness direction retardation Rth of each optical film is set to 300nm. Re is 50 nm similar to FIG. 26.

The slow axis azimuths of four optical films are set parallel orperpendicular to the absorption axes of the polarizers. Morespecifically, in sample S51, the slow axis azimuths of the optical films40 to 43 are set to 45°, 45°, 135° and 135°, respectively. In sampleS52, the slow axis azimuths of the optical films 40 to 43 are set to45°, 135°, 45° and 135°, respectively. In sample S53, the slow axisazimuths of the optical films 40 to 43 are set to 135°, 45°, 135° and45°, respectively.

FIG. 32 is a graph showing simulation results of a transmittance withrespect to a viewing angle, at Δnd=1500 nm and Rth=300 nm. The abscissarepresents an inclination angle of −80° to +80° to the right and left ofa viewing angle from a substrate normal direction, and the ordinaterepresents a transmittance in the unit of “%”. A transmittance becomeslowest under the conditions of 135°, 45°, 135° and 45°. As compared tothe characteristics of the sample S4 shown in FIG. 27, it can beunderstood that an angle range of a low transmittance becomes narrow.Since it is apparent that as the retardation Δnd of the liquid crystalcell increases, the transmittance at a deep viewing angle rises, it canbe estimated that the maximum Δnd capable of obtaining good viewingangle characteristics is about 1500 nm.

A liquid crystal display device having the optical films with theoptimum slow axis azimuths in the simulation is actually manufacturedand an exterior observation is conducted. Similar to the simulationresults, optical leak at a deep viewing angle is suppressed to somedegree. As compared to Δnd=500 nm, it has been found that the degree ofoptical leak is considerably large.

Although the description uses the case that optical films are made ofbiaxial films, good viewing angle characteristics can be realized evenif films having positive uniaxial optical anisotropy at Nz=1, so-calledA plates, are used.

A total of thickness direction retardations Rth of all optical filmspreferably satisfies the following condition: 1·Δnd>total of Rth>0.5·Δndwith respect to the retardation Δnd of the liquid crystal cell.

In the above-described eighth embodiment, C plates are not used in viewof cost reduction. C plates may be used in combination. As indicated bybroken lines in FIG. 26, a C plate 45 may be inserted between the secondoptical film 41 and liquid crystal cell 20, a C plate 46 may be insertedbetween the liquid crystal cell 20 and third optical film 42, or boththe C plates may be used.

Ninth Embodiment

FIG. 33 is a block diagram showing a display apparatus using the liquidcrystal display unit of the first to eighth embodiments. A liquidcrystal display unit 80 includes a plurality of common electrodes 81disposed in parallel and a plurality of segment electrodes 82 disposedat a right angle relative to the common electrodes 81. A cross point ofthe common electrode 81 and segment electrode 82 constitutes a pixel.

A drive circuit 90 includes a segment output circuit 92 and a commonoutput circuit 91. The common output circuit 91 supplies a commonvoltage to the common electrode 81 via a common bus 93. The segmentoutput circuit 92 supplies a segment voltage to the segment electrode 82via a segment but 94. The drive circuit 90 multiplex-drives the liquidcrystal display unit 80. If a potential difference applied between thecommon electrode 81 and segment electrode 82 of a pixel is equal to orlower than an off-voltage, the pixel is a black state, whereas if thepotential difference is higher than an on-voltage, the pixel is a whitestate.

By adopting the structure of the above-described embodiments for theliquid crystal display unit 80, the viewing angle characteristics of theblack state can be improved.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It will be apparent to those skilled in the art that othervarious modifications, improvements, combinations, and the like can bemade.

1. A liquid crystal display device comprising: first and secondpolarizers in cross Nichol configuration; a liquid crystal cell which isdisposed between the first and second polarizers, and which establishesvertical alignment in a state of no voltage application; and an oddnumber of at least three optical films which have optical anisotropy,and which are disposed between the liquid crystal cell and the firstpolarizer, wherein: a retardation of the liquid crystal cell is equal toor larger than 550 nm and equal to or smaller than 1500 nm; and each ofthe optical films satisfies nx>ny≧nz, where nx, ny and nz are x-, y- andz-components of a refractive index in which an x-axis is an in-planeslow axis azimuth of each of the optical films, a y-axis is an in-planeazimuth perpendicular to the x-axis and a z-axis is a directionperpendicular to a film surface, an in-plane retardation of each of theoptical films is equal to or larger than 30 nm and equal to or smallerthan 300 nm, a thickness direction retardation of each of the opticalfilms is equal to or larger than 50 nm and equal to or smaller than 300nm, an angle between an in-plane slow axis of the optical film disposednearest to the first polarizer and an absorption axis of the firstpolarizer is equal to or larger than 45° and equal to or smaller than135°, and the slow axes of mutually adjacent optical films areperpendicular to each other.
 2. The liquid crystal display deviceaccording to claim 1, wherein a difference between the in-planeretardation of each of the optical films and an average of the in-planeretardations of the optical films is equal to or smaller than 10 nm. 3.The liquid crystal display device according to claim 1, wherein theoptical films have a same thickness and a same refractive indexanisotropy.
 4. The liquid crystal display device according to claim 1,wherein a total of the thickness direction retardations of the opticalfilms falls in a range between 0.5 times and 1 time the retardation ofthe liquid crystal cell.
 5. The liquid crystal display device accordingto claim 1, further comprising a drive circuit for multiplex-driving theliquid crystal cell.